Were there some surveys conducted in the aftermath of the 2017 UK election (or even right before) that tried to find out what were the main issues that voters had on their mind in making their choice? I'm not looking merely for pundits' opinions on the matter... but I'm aware of some possible reasons outlined in the press, e.g. the proposed "dementia tax" supposedly having been a turning point:

The key turning came on May 19 — when the Conservatives released their manifesto, the British equivalent of a party platform. That day, the Conservatives led Labour by 47.2 to 31 — a 16.2-point margin. The margin started dropping almost immediately after that day, falling more than 6 full points, down to roughly 10.1 (44.8 to 34.7).

The key problem with the Conservative manifesto was a proposal to require individuals who need in-home support services, like a nurse, to pay for these services on their own if their combined savings and assets, including property, total 100,000 pounds (roughly $130,000) or higher. Currently, the UK’s ”social care” system pays for this kind of in-home assistance for many more people than would be covered under the Conservatives’ plan.

The proposal, which reportedly was added to the manifesto at the last minute, was an immediate disaster. Critics dubbed it the “dementia tax,” as many people who rely on social care are elderly individuals afflicted with dementia. It came across as unnecessarily cruel, once again playing into a longstanding sense that the Conservatives aren’t really interested in helping Britain’s most vulnerable.

The criticism of the “dementia tax” was so overwhelming that, three days later, it was removed from the Conservative manifesto. This actually managed to make things worse: It suggested that May, who had been running on the slogan “strong and stable,” wasn’t actually to be trusted.

This sounds fairly reasonable as an explanation, but it is not the only contestant:

It might have been possible for May to salvage this debacle if she sold voters on her own personal qualities. But she was horrifically wooden on the campaign trail; the word “robotic” was commonly used in the British press to describe her performance. She didn’t really speak to large audiences and rallies, and refused to participate in a debate among leadership candidates.

Corbyn, by contrast, held big rallies showcasing his ability to work a crowd. The Labour leader actually performed relatively well in high-profile media interviews, like a live grilling from popular TV host Jeremy Paxman. This steady performance in the face of Conservative blunders made him seem like a fresher choice than most people thought.

So are there reasons that can be found in population surveys for why the 2017 UK election went the way it did?

I found some of the usual demographic correlations in a YouGov suvery, namely (lower) age and (higher) education being the main predictors of a vote for Labour, but I think that's not particularly insightful in the context of this election.

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YouGov did some exit polling that broke down some of the motivators for voting in the 2017 UK election. While it doesn't break it down by policy, it does give policy-based motivations among a list of others. The largest motivator for Conservatives was Brexit, while the largest motivator for Labourites was the Labour Manifesto. Lord Ashcroft, a well-known pollster, presented a different list of reasons with similar results. Conservatives and Lib Dems voted largely on Brexit, Labour voters largely on the NHS and Spending cuts. I highly recommend Ashcroft's post-election analysis, which goes deep into the specifics of the motivators for voting in the 2017 UK election.

Apparently youth turnout rose faster than the elder's, and given the youth's inclination to vote Labour, that has some explanatory power.

Because turnout increased in many younger areas, which often had large numbers of students,
many commentators talked about a ‘Youthquake’. Of the 20 constituencies in England with the
highest concentration of 18-29 year olds, turnout increased by an average of 4.6 percentage
points but of the 20 seats with the lowest proportion of young people, turnout increased by just
2.6 points. Thus, even though ‘older’ seats still tended to record higher turnout, in 2017 it was
often younger seats that recorded the sharpest increase in turnout on the previous election in
2015.

What is a bit more surprising is that Labour also made gains in some pensioner-heavy areas:

Of the 20 seats with the
largest proportion of pensioners the Conservative Party vote increased by 7.6 percentage points
while Labour’s increased by almost 9 points. Yet in the 20 seats with the lowest proportion of
pensioners the Conservative vote declined by 2.4 points while Labour’s vote surged by 12.

So perhaps some of "dementia tax" affair had indeed affect some older voter's choice. (This is my opinion, the "dementia tax" affair is not mentioned in the study.)

May's bet of capturing UKIP voters/seats mostly paid off, but she had much less success in capturing Labour seats that voted Brexit. This is perhaps the most interesting part of the analysis. Based on some regression models relating policy preferences to actual vote, the study concluded that

Other things being equal, support for Labour among people on low incomes with left wing
economic views was 66% compared to just 23% for the Conservatives – representing a
Labour lead of over 40 percentage points.x By contrast, support for Labour among people on low
incomes who are pro-Brexit was just 32% compared to 54% for the Conservatives –
representing a Tory lead of 22 percentage points.xi This indicates that the effect of support for
left wing economic views had a stronger impact on vote choice than the effect of pro-Brexit and
anti-immigration views. Even though the Conservatives did well among the segment of the
population on low incomes who were relatively pro-Brexit, Labour did even better among the
segment who were relatively left wing. To have the best of both worlds people on low incomes
would probably favour a party that offered them both redistribution and control of immigration,
but given the choice on balance their preference for redistribution outweighed their preference
for immigration control.